Recently, more and more strict demands are asked on the DC/DC converter such as high efficiency, high power density, high reliability and low cost. Please refer to FIG. 1, which is a block diagram showing a dc/dc converter according to the prior art. In FIG. 1, the dc/dc converter 10 includes a converting stage circuit 11, a diode-rectifying stage circuit 12, and a filter and load stage circuit 13. The operation principle of the dc/dc converter 10 includes the steps of: a dc voltage Vin is firstly modulated by the converting stage circuit 11, then rectified by the diode-rectifying stage circuit 12, and finally filtered by the filter and load stage circuit 13 to be sent to a load (not shown).
In the dc/dc converter 10, the energy is delivered from the converting stage circuit 11 to the filter and load stage circuit 13, which is a uni-direction path. And sometimes this “uni-directional path” energy transferring method will cause the output voltage of the dc/dc converter 10 unstable when operating at light or no load condition, as shown in FIG. 2(a).
Please refer to FIG. 2(a), which is a circuit diagram showing a full-bridge LLC converter according to the prior art. The full-bridge LLC converter is generally operated using Pulse Frequency Modulation (PFM) technique. In FIG. 2(a), the full-bridge LLC converter 20 includes a converting stage circuit, a diode-rectifying stage circuit, and a filter and load stage circuit. The converting stage circuit includes four switches Q1˜Q4, a resonant capacitor C1, a resonant inductor L1, a magnetizing inductor L2, and a transformer T1. The diode-rectifying stage circuit includes two diodes D1˜D2. The filter and load stage circuit includes a filter capacitor Cout. The switches Q1 and Q2 constitute one bridge arm and the switches Q3 and Q4 constitute the other. The respective driving signal of the switches Q1 and Q4 and the switches Q2 and Q3 drives the switches at nearly 50% duty cycle. Between the midpoint of the two bridge arms are the resonant capacitor C1, the resonant inductor L1, and the primary side of the transformer T1, which are connected in series. The secondary side of the transformer T1, which is a center-tap structure, includes two diodes D1 and D2 to form a full-wave rectifier. The output side of the full-bridge LLC converter 20 includes a capacitor Cout for filtering and stabilizing the output voltage.
For a resonant converter with the diode rectifying technique, there exists a minimum voltage gain in the range of the operation frequency thereof, for example, the minimum voltage gain obtained when the above full-bridge LLC converter 20 is operated at the highest operation frequency. Generally, a converter is designed to have its gain more than the above minimum voltage gain when operating in the range of the operation frequency thereof, and the converter is theoretically able to be operated stably with complete zero load. In practice, due to the parasitic oscillation generated by the parasitic parameters of the elements, e.g. the parasitic capacitor at the primary or secondary side of the transformer, an excess of energy will be injected into the output terminal so as to cause the output voltage to rise when using the diode-rectification at the secondary side, as shown in FIG. 2(a). Thus, the converter will be unstable when operating at light or no load condition.
To solve the aforementioned problems, there are at least four technical schemes in the prior art, which are provided as follows.
The first is to consume the excess energy injected into the output terminal. The practical method is to install an adequate dummy load. However, the dummy load will cause the converter to be operated in a lower efficiency and consume more power when operating at no load condition. Besides, the size and the cost are also increased, too.
The second is to install an independent auxiliary circuit. When operating at light or zero load condition, the main circuit is switched off and the auxiliary circuit is operated to maintain the output voltage. In this regard, there will be no additional loss at normal load. However, it needs load judgment additionally and switching between the auxiliary circuit and the main circuit, which increases the control complexity and adds the additional requirements on the dynamic performance of the converter.
The third is to adopt burst mode control technique to reduce the energy transferred from the input terminal to the output terminal when operating at light load or no load condition.
The fourth is to prevent the excess energy from being injected into the output terminal when operating at light load or no load condition, which is realized by changing the resonant parameters or the resonant impedance. There are at least three methods as follows:
(1) U.S. Pat. No. 5,388,040
Please refer to FIG. 2(b), which is a circuit diagram showing the full-bridge LLC converter provided in U.S. Pat. No. 5,388,040. In the full-bridge LLC converter 21, the elements which are the same as those in FIG. 2(a) are marked with the same numerical symbols.
The technical scheme adopted in U.S. Pat. No. 5,388,040 is to change the resonant parameters according to the load conditions. As FIG. 2(b) shows, a switch Sa is introduced into the main circuit to be connected with the magnetizing inductor L2 in series. The equivalent magnetizing inductance is able to be adjusted by controlling the switch Sa. When operating at light or no load condition, the equivalent magnetizing inductance of the main circuit will be decreased after the switch Sa is turned on. Therefore, the minimum voltage gain of the main circuit will also be decreased in a specific range of operation frequency. Thus the main circuit will be operated stably.
(2) JP Patent No. 8,033,329
Please refer to FIG. 2(c), which is a circuit diagram showing the full-bridge LLC converter provided in JP Patent No. 8,033,329. In the full-bridge LLC converter 22, the elements, which are the same as those in FIG. 2(a) are marked with the same numerical symbols.
The technical scheme adopted in JP Patent No. 8,033,329 is to change the resonant impedance at different load conditions. As FIG. 2(c) shows, a parallel resonant unit composed of an inductor L2 and a capacitor C2 is in the resonant loop constituted by the resonant capacitor C1, the resonant inductor L1, and the primary side of the transformer T1, so as to increase the impedance of the resonant loop when the converter 22 is light- or zero-loaded. Thus the whole system will be operated stably accordingly. However, the drawback is that the parallel resonant unit will bear a large voltage current stress when the main circuit is light- or zero-loaded.
(3) JP Patent No. 2,106,164
Please refer to FIG. 2(d), which shows the full-bridge LLC converter provided in JP Patent No. 2,106,164. In the full-bridge LLC converter 23, the elements, which are the same as those in FIG. 2(a) are marked with the same numerical symbols.
As FIG. 2(d) shows, a series circuit composed of an auxiliary switch S and a resistor R is connected to the resonant capacitor C1 in parallel. With the series circuit, the energy at the resonant capacitor C1 will be consumed when operating at zero load condition, in order to prevent the excess of energy from being injected into the output terminal.